<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button class='btn'> normal</button>
    <button class='btn'>primary</button>
    <button class='btn primary'>else</button>
    <style>
        .btn{
            display: inline-block;
        }
        .btn.primary{
            color: #fff;
            background: #218de6;
        }
    </style>
</body>
</html>